1. Technical Field
The present invention relates to time delay elements, and more particularly to an optical apparatus for producing variable time delays in an optical signal.
2. Discussion
The ability to provide adjustable time delay in a signal is required in many applications. For applications where the signal is a continuous wave, effective time delay can be realized by inducing a phase shift, .phi., in the signal of between 0 and 2.pi. radians. Long time delays can still be achieved for a single frequency continuous wave signal by phase shifting. Any arbitrarily large phase shift is strictly equivalent to a phase shift between 0 and 2.pi. for that specific case. However, two important classes of applications cannot use phase shifting techniques to achieve time delays corresponding to more than a few cycles. These are continuous wave signals with finite bandwidth and pulsed digital applications. For these types of applications, true time delay is necessary and phase shifting techniques will not suffice.
One method of generating true time delay in an optical signal consists of a set of optical fibers of different lengths, with a switching mechanism to select the fiber with the proper length. These types of delay generators have significant problems associated with the large insertion loss of the "one to many" switching mechanism and the large number fibers necessary for achieving a wide range of optical delay. Further, such delay units require active switching which requires active control signals and drive electronics that add to the complexity bulk and cost of the system.
One example of the use of delay elements is in RF phased array antennas. Fiber optic cable is increasingly being considered as a replacement for conventional microwave guide and coaxial cable in such feed networks because optical fiber offers a number of advantages over conventional feed networks. These advantages include lower weight and size, as well as less signal attenuation at high RF frequencies. An important feature of phased arrays is the ability to provide adjustable true time delay to the signals being fed to the various antenna elements so that the proper phasing results between these elements for all frequencies in a broad band antenna.
Phased array antennas that rely on phase shifting only, can provide the proper equivalent delay at only a single frequency, severely limiting the bandwidth capabilities of the system. Further, such phased array antennas display "squint", a phenomena where the antenna points in different directions for different frequencies within the bandwidth of the antenna.
Conventional phased antenna systems achieve time delay of the RF signal feeds by using a combination of electronic phase shifters and switching elements which select different lengths of RF waveguide sections according to the required delay time. These same methods can be carried over to fiber optic feed systems, again using electronic phase shifters for fine control (sub RF wavelength) and a fiber optic switching matrix for primary control of time delay. Optical fiber segments of different lengths are switched into each feed path as in conventional RF waveguide networks to achieve the desired time delay. However, such switching matrices have the disadvantages discussed above.
In addition to the phased array antenna application, similar problems occur in optical processors relying on coherent techniques such as phase quadrature. Phase shifters will impart the proper shift only for a limited bandwidth of frequencies. Other signal processing techniques requiring substantial time delays, such as auto correlation, will also be severely limited in bandwidth if phase-only techniques are utilized. True time delay is also essential in digital applications such as optical clock distribution in large reconfigurable electronic circuitry. Phase is not well defined in a digital pulse train unless it is of constant frequency, f. Phase shifting of such a pulse train will only work for time delays up to 1/f. Phase shifting of pulse trains is usually accomplished by true time delaying the pulse train.
Thus, it would be desirable to provide an apparatus for producing variable true time delay in an optical signal which does not require multiple optical fibers, nor active switching, and which does not have a high insertion loss. Accordingly, it would be desirable to provide a technique for producing true time delay in an optical signal which is generally passive in operation. Providing these features in an apparatus which is not limited in RF bandwidth would also be desirable. In addition, it would be desirable to provide such a true time delay element which is relatively simple, compact and inexpensive.